Adjustable height platform supports are well known in the art and typically comprise four main elements, namely an upright, a bracket assembly for supporting a platform, a jack or block and tackle for raising and lowering the bracket assembly on the upright, and a means for holding the upper end of the upright in place.
Two types of adjustable height platform supports with uprights constructed of wood or rubber-backed aluminum are well known and widely used.
An example of an adjustable height platform support with the upright constructed of wood is shown in Hoitsma U.S. Pat. No. 2,216,912. The Hoitsma patent discloses a bracket assembly in which a jack is used to raise and lower the bracket assembly. This type of bracket assembly is referred to as a “Pump Jack”. Another example of an adjustable height platform support with a wooden upright is shown in U.S. Pat. No. 2,342,427 to Henry J. Riblet (herein after “Riblet'427 patent”). The Riblet '427 patent discloses a bracket assembly, which is raised and lowered by block and tackle. This type of bracket assembly mounted on an upright has been commonly referred to as the “Painter's Pole”.
Examples of adjustable height platform supports, which use an aluminum upright, to which a rubber strip has been riveted, include U.S. Pat. No. 4,597,471 to Anderson and U.S. Pat. No. 5,259,478 to Berish et al. It should be noted that the Anderson patent and the Berish patent both adapt aspects of the Hoitsma pump jack mechanism.
In the Painter's Pole bracket assembly, an inner jaw of a shackle of the Painter's Pole bracket assembly is located below a plane containing both the fulcrum axis and an outer jaw of the shackle. A load on the angle bracket of the Painter's Pole bracket assembly provides a turning movement on the shackle, which activates the shackle by bringing both jaws, namely the outer jaw and an inner jaw of the shackle into contact with the upright when the width of the upright exceeds a “design width”. Accordingly, a shackle in which the inner jaw lies below the plane through the fulcrum axis and-outer jaw will be referred to as a ‘load activated shackle’.
It is noted that the shackle of the pump jack bracket assembly and shackle of the painter's pole bracket assembly are similar in that they have two parallel side members, which support the two jaws. The two jaws function to directly contact the upright and transfer forces acting on the angle bracket to the upright.
The two parallel side members are indirectly pivotally coupled to the load supporting angle bracket for the pump jack and directly pivotally coupled to the load supporting angle bracket of the painter's pole by means of the fulcrum bar. In both cases, the angle bracket may be lowered by rotating the shackle about the fulcrum bar in a downward direction. In practice, the aforementioned is advantageous in that an operator may lower the angle bracket if the operator depresses the shackle, directly or indirectly, with his foot. Of course, a heavy object falling on the shackle can cause an accident.
Although pump jacks and painter's poles are usually used to support a scaffold extending between a pair of adjustable height platform supports so that a workman can move back and forth between the uprights, there are a number of other applications for these adjustable height platform supports.